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Sparking Fused Switch

01/30/2017 6:49 AM

A friend of mine asked me to look at a fused switch for a water heater (uk domestic 230V) in his kitchen. It was installed by an electrician and he just noticed it was causing a large spark when switching it off. Not when switching it on.

I suggested just changing the switch. To my (limited) electrical knowledge it would either be the switch or the heater is drawing too much current and a new switch is inexpensive. Anyway, the new switch also sparks but only when switching off.

Why does it not spark when switching on and if the heater is drawing too much current then why isn't the MCB tripping? Or is there another reason why this is happening?

Oh, and I advised him to have an electrician look at it so no health and safety concerns here.

Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting as Little England and Wales (not too sure about Wales bit, either). Kettle's on.

Re: Sparking Fused Switch

01/30/2017 7:03 AM

<...too much current...> Were that the case, then the circuit protective device would have operated to disconnect the fault and protect the wiring!

A UK domestic heater will have a current draw not exceeding 13A. Opening the switch while the heater is energised causes a momentary arc as the contacts begin to separate, which is quickly extinguished as the current drops back and the contacts continue to separate.

If the installation has been installed by a qualified Electrician then there will be a test certificate document to hand that warrants that the installation was safe when the Electrician had completed the work. If in doubt, have the same Electrician revisit the installation for re-assessment under warranty.

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Re: Sparking Fused Switch

01/30/2017 12:52 PM

First of all, there is likely to be a little bit of "sparking" going on when the switch is turned on, only, it just isn't as exciting. The interruption of the circuit, however, sets up a condition where the voltage can climb very rapidly and then it becomes more noticeable. That is likely due to the fact that a fair amount if inductance is present somewhere in the circuit, including the power transformer being used to power the water heater. (especially if it is isolated from other loads)

Because the voltage can spike when the current is interrupted (V= L(+R)*di/dt) the spark is initiated very rapidly and it ionizes a bit of gas around the separation of contact points in the switch. Ionic gas is conductive so it may briefly assist in the attempt to maintain the current, resulting in an even larger spark effect.

A very common fix might be to have another load present and connected while the water heater is being disconnected. Yes, it will dump the residual energy into the other device, but the spark will be less noticeable. Or, just buy a zero crossover detection disconnect device. Disconnecting while the voltage swings through 0 V will generally result in no spark being created.

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Re: Sparking Fused Switch

01/30/2017 1:14 PM

Simpler solution. When he wants to disconnect it, turn the temperature setting all the way down first so that the heater is not actually trying to heat water when the switch is opened. No load, no current flow, no sparking.

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Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting as Little England and Wales (not too sure about Wales bit, either). Kettle's on.

Re: Sparking Fused Switch

It probably wouldn't be as visible inside the thermostat as they are generally a sealed unit, but I wouldn't fancy having to access the innards of the heater every time I wanted to turn it off.

White switches generally tend to display the spark far more vividly than do darker coloured ones, so one alternative may be to simply fit a brown switch or a heavier duty one that incorporates an inner mechanism that hides the flash from view (thinking a weatherproof type or similar here).

Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting as Little England and Wales (not too sure about Wales bit, either). Kettle's on.

Re: Sparking Fused Switch

01/30/2017 10:57 PM

Until the switch is actually closed there is no current in the circuit and therefore no interruption to the current flow. It is the interruption to flow which causes the arc, and in an AC circuit that arc will rapidly be extinguished as the sine wave goes through the zero crossing and the resultant current also momentarily falls to zero. It does this 100/120 times per second for a 50/60Hz supply respectively, so the arc will be of very short duration and would normally not be considered to be of concern.

The heavier the load (the higher wattage the element) the larger the spark will normally be, but sometimes you may see very little spark if the opening of the switch happens to coincide with, or be very close to, the sine wave zero crossing.

A water heater element is almost entirely resistive with a very minor inductive component, so you can discount any theories regarding inductive field collapse etc.

Re: Sparking Fused Switch

02/03/2017 12:12 PM

I have to disagree with you on your comment about the inductive component. Sure the load is resistive. But the load is only part of the circuit. Surely you don't consider the transformer from which the power comes to be resistive?

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A great troubleshooting tip...."When you eliminate the impossible, whatever remains, however improbable, must be the truth." Sir Arthur Conan Doyle

Re: Sparking Fused Switch

An upstream supply transformer is an entirely different scenario to a switch breaking a downstream inductive load where the full transient is directly impressed on the switching medium.

The transformer supplying a residential area is likely to be a pretty big unit supplying many thousands of amps across multiple phases at any one time.

The disconnection of a tiny fraction of one percent of its load is not going to affect its magnetic field by any measurable amount, and any impulsive transient that were to be produced by any minor field collapse in the transformer would be absorbed by all of the other equipment on line at the time and would not in any case be impressed upon the switch contacts.

Probably the easiest and cheapest way to limit arc damage at the switch is to use a double pole switch and connect both sets of contacts in series, this splits the arc into two separate parts, one at each set of contacts thereby significantly reducing the level of ionising heat that can be produced by each arc.

Re: Sparking Fused Switch

01/31/2017 5:36 AM

Why is he switching it on and off? Presumably the heater has a built-in thermostat so most people leave it switched on. If he does want to switch it off, do it when the stat is satisfied and there will be no spark.

In any case, some sparking is normal and it's unlikely to be a problem, as others have said. I'm surprised you can see it, is the switch in a translucent housing?

Re: Sparking Fused Switch

01/31/2017 8:46 AM

Sounds like the current "green" thought process - turn off water heaters when not at home, because the heating cycling is not needed then. Turning back the thermostat is the normal way of doing this, but someone has found that switching the power to it is easier. Now we need to figure energy saved by turning this off, versus energy spent making replacement contacts when they burn through way too soon.

I bet he also turns his home heating system back every day when he leaves for work.

That is the problem with so many "green" suggestions - yes the suggestion appears to save energy, but so many have not been researched the whole way through the process and possible consequences.

Re: Sparking Fused Switch

01/31/2017 5:46 AM

After reading the replies I noticed that no one mentioned the eventual erosion of the contacts due to the sparking. The contacts will eventually wear out and need to be replaced. Has anyone checked that the fuse is actually a 13 amp one. As stated elsewhere, there is very little chance of there being any inductive component to the circuit (except for isolation transformers and magnetic coils in the switch). Take the advice and get it checked by the installer. B t w, are there any flammables around, there often are in areas around water heaters. All of this in m h o of course.

Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain, and now disconnecting as Little England and Wales (not too sure about Wales bit, either). Kettle's on.

Re: Sparking Fused Switch

A typical will have contacts rated at 13A and be type-tested by design for this sort of purpose, which is why there are Nx106 in use.

If the switch is a then these are only rated for use on lighting circuits at lower currents, typically a 6A circuit, and one would not expect the thing to last long at 13A.

Fortunately for the original poster, a qualified has installed it, so it is more likely from a to be a and not a , otherwise the would not have been able to it in accordance with part P of the UK's Building Regulations in order to satisfy the end user's company. Regardless, the original poster still has a remedy under as a result of using a qualified and not a .

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Re: Sparking Fused Switch

Re: Sparking Fused Switch

02/10/2017 6:41 PM

That a good explanation

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Re: Sparking Fused Switch

01/31/2017 9:09 AM

The switch is indeed the 13 amp one shown in the last post. It was installed by a professional electrician.

My mate isn't switching it on and off all the time. He is actually doing up the flat for rental and noticed (a good few days after the electrician installed it) that when he switched it off there was a very noticeable spark. Because he isn't living in the flat just now he switched the heater on to get hot water then switched it off as he wouldn't be back for another week.

I've also changed plenty of switches which were old and causing a small spark but this is rather a large spark and my question was really that I could only think of two reasons why it would do that. Either the heater was drawing a large current, but then why was nothing tripping, or the switch was faulty. So I changed the switch for him, for a brand new one, and it still sparks. You can hear a pop and you can see the spark between the housing and the toggle.

Re: Sparking Fused Switch

01/31/2017 9:40 AM

Tried to open your link but got a "Harmful website" warning so didn't risk it.

Could it by any chance be an instantaneous type heater? A quick google shows plenty, rating 9.5 - 12kW or more. That would draw a much bigger current and give a bigger spark. Obviously the switch would need to be rated to suit.

But, thinking about it some more after posting, that type would have a switch that closes when water flow is detected, open when flow stops, possibly also on high temperature. So it's unlikely to be any current when the main switch is opened.

Re: Sparking Fused Switch

Re: Sparking Fused Switch

02/01/2017 4:18 PM

Seems like everybody forgot that the cold (off) resistance of a heating element is much lower than the hot (on) resistance; therefore the "make" current is much higher than the "break" current. The spark you're seeing is the metal surface at the initial point of contact overheating and vaporizing, which can ultimately lead to the contacts welding together.

The way to minimize this is to use a "T-rated" switch which is specifically designed to handle the high inrush currents of the cold tungsten filaments of many parallel incandescent lamps that (used to be) found in commercial/industrial installations that didn't warrant the use of a much more expensive lighting contactor.

Another factor is the specific construction of the switch since there are many types out there; here's a good article on their differences. nb- I am only familiar with devices used in the USA.

Re: Sparking Fused Switch

02/01/2017 8:36 PM

Assuming that the element is Nichrome, it would likely be either type A or C .

From a base of 20°C. Ni Cr A increases in resistance by roughly 6% at 600°C, and Ni Cr B by about 9%. Above that temperature, type A resistance drops slightly and then rises back to about 6% at 1000°C whereas type C continues to rise slowly to about 10% at 1000°C

An element that draws say 15 amps at 600°C will therefore have a current draw of either 15.9 amps or 16.3 amps when cold dependent on the wire type, and a water heater element doesn't have the vast operating temperature range as does say a stove element, so the Ohmic difference between cold and maximum heat in water would be quite small.

I would have to doubt that the switch rating would be compromised by this difference, but I agree that the type of switch employed is important both for longevity and operator confidence.

Re: Sparking Fused Switch

02/02/2017 10:48 AM

Where large currents are involved, it is easy to create sparks or arcs when the current path is interrupted. The design of switch contacts adds mass to the contact to help absorb the energy of the arc; at the expense of occasionally surface melting tiny portions of the contacting surfaces. These tiny melts can build up and cause intermittent open failures in the switch because the surface of the contacts are no longer smooth. A low cost protection for a switch contact is an energy absorbing component often referred to as an MOV. these are available in a wide variety of energy capacities and are relatively low in cost compared to a switch. The MOV is installed across the contacts of the switch, and when the switch is opened the arc will be absorbed by the MOV. Ask your electrician to select the proper voltage and energy value, and install the MOV accordingly.

Re: Sparking Fused Switch

02/02/2017 5:11 PM

As this particular circuit is almost entirely resistive, the voltage across the opening contacts will not be significantly higher than the line voltage, a MOV (Metal Oxide Varistor) will not work as its clamping voltage would have to be so close to the actual line voltage that it would be in danger of spurious misfires.

If this circuit was inductive, then a MOV would be a good choice, but it would be placed across the load rather than across the contacts as you suggest.

Re: Sparking Fused Switch

02/03/2017 3:08 AM

Thanks for the replies. My friend told me last night the electrician had a look and said there is nothing wrong with the switch however the current drawn when the heater is switched on is close to the maximum the switch can take. Once the water heats up if you switch it off it doesn't spark.

If it were me I'd uprate the switch so it stopped the spark or at least reduced it but the electrician says its fine and just leave it. He's also signed it all off so my mate is covered from a legal point of view.